Liquid crystal devices are the dominant technology presently used as light modulators in flat-panel displays. They operate by a shuttering principle in which selected pixels of the display are chosen--normally by application of an electric field across the liquid crystal layer--either to block or to transmit light. This occurs through a change in the optical properties of the liquid crystal material, often in combination with a static optical element such as a polariser. Thus a pattern of dark and bright pixels can be formed dynamically. The light required for an image to be seen can either be provided by a light source behind the liquid crystal shutter (i.e. the other side of the liquid crystal layer from the observer) or in front. In the latter case the light is either blocked, or transmitted and then reflected back to the observer. In general for graphic displays (high information content with a large number of pixels, typically &gt;10,000), the light source takes the form of a backlight to the liquid crystal device. The power efficiency of the display is determined by a combination of the efficiency of the backlight, and the efficiency of transmission through the liquid crystal shutter when it is open. For colour displays, where red, green and blue absorbing filters are used to provide individual colours to each pixel, the typical efficiency overall is about 2 to 5%.
Pixels are selectively controlled to transmit or block light by electrically addressing the display. Row and column electrodes are provided which define pixels where they overlap. Row and column drivers apply voltages respectively to the row electrodes and column electrodes.
One of the most common configurations for such flat panel displays is passive matrix addressing, where line by line addressing of a display takes place through application of row selection voltages to select each row in turn while data voltages are applied to the columns. In this way individual pixels in each line are selected by selection of the field applied across each pixel in the line, the field being dependent on the difference between the row selection voltage and the data voltage. Each row in turn is selected until a whole frame has been addressed. Liquid crystal material does not respond immediately to a change in applied field. The time for a liquid crystal to change its state is its response time. Most liquid crystal devices use liquid crystals that have a response time that is of the same order as the frame time That is, by the time a frame has been addressed, selected pixels in the first rows will have changed state. The state is determined by the root mean square voltage V.sub.rms applied during the frame. The idea is that the final state of any pixel will be determined ideally by the column data voltage applied during row selection of that pixel. Thus a differential in V.sub.rms is created between selected pixels and non-selected pixels. One potential difficulty with this scheme is cross-talk between rows, which means that pixels in rows that are not selected may be affected by the column data voltages intended for selected rows--i.e. the field across a pixel in a given row will change even when that line has not been selected. The effect to the viewer of this cross-talk is reduced contrast of the image.
Where the response time of the liquid crystal is faster than the frame time, then by choosing suitable row select and column data (pixel on and off) voltages the number of lines which have been addressed before the liquid crystal changes state is reduced, and thus the cross-talk problem which arises as outlined above diminishes. However, when the liquid crystals are designed for higher speed operation, other problems can arise. For video rate operation of a display then the frame time for a display needs to be less than 20 ms (allowing flicker free presentation of images to the eye). The ratio of the row select to column data voltages required to prevent crosstalk in a display depends on the number of lines--one model proposes a proportional relationship of this ratio with the square root of the number of lines. In a fast liquid crystal, an incorrect response by pixels in a selected row is possible due to the typically large size of the row select voltage to the data voltage ratio for typical line numbers of graphic displays (i.e. over 200). This effect also leads to a reduced viewer contrast. These issues have been studied in detail over the last few years, and are reported in the literature e.g. Electronic Display Devices, ed by S. Matsumoto and published by John Wiley in 1990.
To produce graphic displays at video rate operation with high contrast, the most common approach is the so-called active matrix addressing, where a thin film transistor is placed at each pixel. The transistors are addressed to transmit voltages which can cause the liquid crystal material to change its optical state. The voltage threshold for switching of the transistor is much sharper than for the liquid crystal material and therefore the cross-talk problem does not occur. The problem with this approach is cost.
Another approach (the so-called active addressing scheme) does not require a transistor at each liquid crystal pixel, but relies on simultaneous application of time varying voltages to rows and columns, where the voltage sequence is calculated in real-time by on-board signal processing chips that determine the driver voltages. This is also a high cost approach.
Another approach (so-called multiscan) effectively reduces the number of individually addressed lines in the display by having separate column drivers to divide the screen into different parts so that selected rows are addressable in parallel. This reduces the number of lines per part and so allows a reduction of the row selection voltage, thus avoiding the incorrect response problem described above, but requires extra numbers of column drivers.
Reference is made to WO91/10223 which describes a backlighted LCD display formed by placing a single matrix of LCDs over a bank of red, green and blue fluorescent lamps. That arrangement is intended to improve the efficiency of a colour display, by avoiding the use of absorbing filters in the display itself. It does not however attempt to address the cross-talk problems discussed above.
Reference is also made to WO93/13514 which also describes a colour fluorescent backlight for a liquid crystal display. In that device, a plurality of phosphorescent strips capable of emitting red, green or blue light are disposed in a vacuum chamber-to constitute a backlight. The document discusses the possibility of optical cross-talk between adjacent rows of LCD pixels due to the spread of light from the backlight, but makes no attempt to address the difficulties associated with the problem of electrical cross-talk as outlined above.
It is an object of the invention to provide a display which can be addressed by a passive matrix system with simple drive circuitry but which does not have a cross-talk problem at high frame rates. In this context, "high" can mean for example &gt;10 Hz.
According to the present invention there is provided a display comprising a light modulator defining an array of pixels arranged in rows and columns and addressable to select between at least an opaque state for each pixel and a transparent state for each pixel, the light modulator comprising a layer of liquid crystal material disposed between a first set of row electrodes and a second set of column electrodes, said pixels being defined where the row electrodes and column electrodes overlap; and
a light emitting device arranged adjacent the light modulator to act as a light source for the display and addressable to emit light from selected regions, each region overlapping at least a plurality of said rows, the light emitting device comprising a layer of an organic electroluminescent material arranged between a first substantially planar electrode and a plurality of substantially planar second electrodes, overlapping said first electrode, each second electrode defining with the first electrode one of said light emitting regions.
Thus, the invention provides a backlight which is composed of selectively actuatable light emitting regions that need not all be simultaneously selected. A simple passive matrix display with a backlight can consequently be used without undue deterioration in contrast because the number of lines visible at any time is reduced, only the row s overlapping a selected light emitting region are visible. Cross-talk in the light modulator will not affect the observed image because the backlight behind the prospective cross-talk areas will not be selected,land therefore that part of the image affected by the cross-talk area is not observable. This allows for lower row selection voltages because of the reduced effective line number, and therefore the ratio of row select to data voltage can be reduced thus avoiding the in stantaneous response problem in fast displays. For example, voltages may be &lt;20V for a row select voltage and 2V for column data voltage, giving a ratio of &lt;10.
In the described embodiment, addressing circuitry is provided for sequentially addressing each of a plurality of said rows while one of said regions is selected to emit light. The addressing circuitry comprises column drive circuitry for applying, for each selected row, data voltages on selected ones of said column electrodes; and row addressing circuitry for sequentially addressing said rows. The addressing circuitry further comprises selection circuitry for sequentially selecting said regions to emit light, selection of said regions being synchronised with operation of said row addressing circuitry to address said rows so that after each plurality of rows has been addressed, a next region is selected to emit light.
The light emitting layer can be a semiconductive conjugated polymer such as polyphenylenevinylene (PPV) or its derivatives. Alternatively, it can take the form of an organic molecular film which, when excited, emits light. Suitable organic molecular films are disclosed in C. W. Tang, S. A. Van Slyke and C. H. Chen, J. Appl. Phys., 65, 3610 (1989).
The present invention can be used to advantage in a colour display wherein each light emitting region can be arranged selectively to emit light of a different colour. In one embodiment this is achieved by providing for each light emitting region a plurality of different zones arranged adjacent one another to emit light of respectively different colours which can be selectively actuated. Alternatively, different zones can be arranged overlapping one another (or stacked) to emit light of respectively different colours.
The invention also provides a method of addressing a display comprising a light modulator defining an array of pixels arranged in rows and columns and a light emitting device arranged adjacent the light modulator to act as a light source for the display and having a plurality of selectable light emitting regions, each region overlapping at least a plurality of said rows, in which method a first of said light emitting regions is selected to emit light while each of the plurality of rows overlapped by said light emitting region are addressed in sequence and, when said plurality of rows have been addressed, the first light emitting region is deselected and a next light emitting region is selected.
Thus, a passive addressing scheme can be used while overcoming the cross-talk and response time problems associated with the prior art. A multi-line scan approach is also possible with this invention.